This invention relates to a method and devices using the method for measuring incident gamma-ray energy and direction. More particularly, the invention relates to measuring energy losses and positions of two Compton scatter interactions followed by a measurement of the position of a third Compton scatter or photoelectric interaction to determine the incident gamma-ray energy and direction cone without the necessity for absorbing the full energy of the incident gamma ray.
2. Description of the Related Art
Gamma ray imaging and detection is used in many scientific and commercial applications, including medical imaging, nuclear spectroscopy, and gamma ray astronomy.
One application that makes use of gamma ray imaging is gamma ray astronomy. In the conventional approach two position-sensitive detector arrays are used. The first array uses low-Z scintillation detectors and the second array, separated some distance from the first, uses high-Z scintillation detectors. Gamma rays incident on the first detector array undergo Compton scattering, and the Compton scattered gamma rays must be fully absorbed in the second detector array in order to estimate the energy and direction of the initial gamma ray. This approach has several limitations, including low efficiency, poor energy resolution and poor angular (imaging) resolution.
Alternative concepts to improve on these limitations have been proposed or undertaken that use detectors with improved position and/or energy resolution. However, in all of these approaches the fill energy of the initial gamma ray must be absorbed in order to determine the energy and direction of the incident gamma ray.
Another application is medical imaging using positron emission tomography (PET). In PET, a radio-pharmaceutical positron emitter is administered to a patient. In some PET applications, the radio-pharmaceutical is selected for its ability to preferentially concentrate in a desired tissue, e.g. a tumor. In other applications, the biological uptake or distribution of the radio-pharmaceutical is used to study organ function (e.g. brain or heart). In these applications, PET applies the mechanism whereby the positrons from the radio-pharmaceutical annihilate with electrons to create two annihilation gamma rays, each having an energy of 511 keV, that are emitted in opposite directions at an angle of almost exactly 180 degrees. The gamma rays are detected with position-sensitive detectors and the location of the detections determines a line on which the radioactive decay is located. Multiple events allow a more precise, 3-dimensional determination of the location of the concentration of radio-pharmaceutical and thus the location/morphology of the tumor or the function of the organ of interest. Typically, two identical detectors, each a combination of scintillators and photomultiplier tubes, are required for PET. This instrumentation, however, provides only moderate energy resolution and position resolution capabilities.
Another application is single photon emission computed tomography (SPECT). In SPECT, an injected radio-pharmaceutical emits a single gamma ray per radioactive decay. The direction of the emitted gamma ray is determined by using a collimator in conjunction with a position-sensitive scintillator-photomultiplier detector. The collimator only allows gamma rays from a single direction to reach the detector. This provides a two-dimensional view of the radioactivity. By moving the detector/collimator assembly to view the region-of-interest from many directions, or using multiple collimators and/or detectors, a three dimensional image can be reconstructed. The disadvantage of this technique is that the sharpest images are generated by collimators with narrow apertures. This is very inefficient and hence requires large doses or long collection times.
An application similar to SPECT is planar imaging, which is the traditional form of medical gamma ray imaging, in which a patient is injected with a radio-pharmaceutical as in SPECT but in which the collimator and detector are planar and are not rotated around the patient. The disadvantage is that the image generated is then a 2-dimensional projection rather than a 3-dimensional image of the radiation distribution.
In order to address the disadvantages of the current imaging systems, several concepts have been implemented or proposed that use the Compton scattering process. In Compton scattering, an initial gamma ray scatters off an electron in a position-sensitive detector and the Compton scattered gamma ray, reduced in energy, is detected by a second position-sensitive detector. The angle of scatter is determined from knowledge of the energy loss at the first and second detectors, where it is required that the full energy of the Compton scattered gamma ray is deposited in the second detector. With this information, the direction of the initial gamma ray is restricted to a cone whose axis is the line joining the two interaction sites and whose opening angle is twice the Compton scatter angle. This technique, for example, was the basis for a scintillation-detector imaging gamma ray instrument that was flown on a NASA mission to image the gamma ray sky. That instrument used low-Z and high-Z detectors for the first Compton scatter detector and the second full absorption detector, respectively. Significant disadvantages of this approach are poor detection efficiency and poor imaging resolution, the latter due to the poor energy resolution of the scintillation detectors.
Another proposed concept for an improved Compton imaging detector employs multiple Compton scattering in arrays of position-sensitive silicon detectors. In this approach, the initial gamma ray undergoes several Compton scatters in the silicon detector array, with the initial gamma ray energy either fully absorbed in the silicon array, or the gamma ray escaping the silicon array being absorbed in a scintillation detector surrounding the silicon array. Knowledge of the full energy loss is used, along with the energy losses and positions of the first two interactions to determine the Compton scatter angle at the first interaction site. The most probable interaction sequence for the Compton scatter events is determined from the consistency of the energy losses and scattering angles with the known physics of the Compton scattering process. An alternative to this technique recognizes that the full energy of the incident gamma ray need not be fully absorbed, and proposes that if the interaction order for four Compton scatters and the initial gamma-ray energy are unknown, the initial gamma ray energy and direction can still be deduced.
Another Compton imaging approach employs the use of position-sensitive gas or liquid detectors. A low-Z material (e.g. argon) is used for the Compton scatter detector while a high-Z material (e.g. xenon) is used to absorb the Compton scattered gamma ray. Disadvantages of this concept are the poor energy resolution of gas and liquid detectors relative to solid-state detectors, the low interaction efficiency in gas detectors compared to solid-state detectors, and the associated limitation to low-energy gamma-rays.
There is, therefore, a need for a gamma ray imaging device having improved imaging, improved detection efficiencies, better energy resolution, and capable of extending gamma-ray imaging capabilities to higher gamma-ray energies.
According to the invention, a device for determining the photon energy E1 and direction cone angle of incident gamma ray with two Compton scattering interactions and one subsequent interaction includes a first radiation detector for receiving an incident gamma ray having an unknown photon energy E1 and an unknown direction cone angle, for scattering a photon energy E2 in a first Compton scattering interaction at a first scatter angle xcex81, and for providing a first output corresponding to the first Compton scattering interaction; a second radiation detector for receiving photon energy E2 and scattering some photon energy E3 in a second Compton scattering interaction at a second scatter angle xcex82, and for providing a second output corresponding to the second Compton scattering interaction; a third radiation detector for receiving photon energy E3, and interacting with photon energy E3 in a third interaction, and for providing a third output corresponding to the third interaction; and a processor for receiving and processing the first, second, and third outputs and for calculating the photon energy E1 and direction cone angle of the incident gamma ray based on the outputs. The first, second, and third radiation detectors are sensitive to the position and energy of incident gamma ray, e.g. by virtue of being in a three-dimensional array providing a very accurate determination of the position of a scattering or other event. The device and processor include the capability to calculate the photon energy E1 and direction cone angle of the incident gamma ray without the necessity for the absorption and measurement of the entire or substantially all of the energy E1 in the detector. The detector also preferably includes the processor being programmed to reject a selected detection event, e.g. in an algorithm that also calculates the respective energy losses L1, L2, and L3 in the first, second, and third radiation detectors.
The invention also includes a method for determining the photon energy E1 and direction cone angle of incident gamma ray with two Compton scattering interactions and one subsequent interaction, comprising the steps of receiving an incident gamma ray having an unknown photon energy E1 and an unknown direction cone angle, for scattering a photon energy E2 in a first Compton scattering interaction at a first scatter angle xcex81, and providing a first output corresponding to the first Compton scattering interaction; receiving photon energy E2 and scattering some photon energy E3 in a second Compton scattering interaction at a second scatter angle xcex82, and providing a second output corresponding to the second Compton scattering interaction; receiving photon energy E3, and interacting with photon energy E3 in a third interaction, and providing a third output corresponding to the third interaction; and processing the first, second, and third outputs and calculating the photon energy E1 and direction cone angle of the incident gamma ray based on the outputs, without necessarily absorbing and measuring the entire or substantially all of the energy E1.
The invention provides substantially improved efficiency and imaging resolution compared to current systems.
The invention further provides a system that produces gamma-ray images having improved spectral resolution.
The invention is useful in military applications, for example for the location of fissile materials or for the location and identification of radioactive waste materials. The system and method also has non-military applications, for example in nuclear medical imaging and non-destructive testing, to name but a few.
The invention may have specific application in certain nuclear medical imaging applications, such as that described in pending U.S. patent application No. 09/854,467 filed on May 15, 2001, entitled xe2x80x9cCoincident Multiple Compton Scatter Nuclear Medical Imagerxe2x80x9d.
Additional features and advantages of the present invention will be set forth in, or be apparent from, the detailed description of preferred embodiments which follows.